All of those cards support OpenCL. Vegas uses OpenCL to enhance the calculation speed of the CPU. Certain effects and codecs from the point of decode, to timeline, to encoding can have the math results returned faster by using OpenCL. OpenCL creates a a virtual math co-processor, by combining registers on the CPU with the GPU chip. Unlike multi core CPUs, not every PC process requires OpenCL so your software has to be OpenCL aware. So you would want to choose the device with the best GFLOP performance.

This is complex topic that has been discussed before about 100 times since VP11 came out. Basically the design of your motherboard, amount of RAM, and the speed, amount of cores, and instruction sets of your CPU matter more than your GPU. If your system is running slow ram vegas will be waiting. If your CPU is slow, your fast GPU will look unutilized, since it can complete its tasks so much faster. If you are running a motherboard like an X99, then there is enough CPU side bandwidth to operate multiple GPUs.

"GPU encoders" are dead. Fast CPU+GPU hybrid is a much faster way to go with better results. People that complain GPU does not work in Vegas normally have something else wrong with their configuration. The people that have a stable working configuration generally do not chime in on how much the GPU adds to the mix.